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DOCSIS
DOCSIS Standard Introduction to DOCSIS DOCSIS, abbreviation for Data Over Cable Service Interface Specifications, is a family of standards established by Cable Television Laboratories at the end of 1995 6. The standards specify interoperability for a cable system which consists of an architecture known as hybrid fiber coax (HFC) 4. A cable system typically contains a number of cable modems (CM) located in subscribers side and a cable modem termination system (CMTS) connected through a CATV plant, which is shown by architecture in Figure 1 5. Architechture of DOCSIS.jpg|linktext=Figure 1. Architechture of DOCSIS system. DOCSIS Protocol Stack.jpg|linktext=Figure 2. DOCSIS protocol stack. DOCSIS Protocol Stack The DOCSIS protocol stack specifies a set of protocols that allows how DOCSIS CM to enable subscribers to use Internet. DOCSIS protocol stac is depicted in Figure 2 6. The lower four layers in the protocol stack allows communications over HFC network to be carried out between CMs and CMTS. The upper layer protocols including TCP/IP, UDP and etc are well known protocols associated with the ordinary Internet. Despite the four layers of protocols that differs from the ordinary Internet network, Internet browsing happens the same as usual. In other words, the lower layers are hence hiding from the higher layer protocols. The functionalities of the lower four layers within the protocol stack are briefly summarised in Table 1 6. DOCSIS Versions and Performance DOCSIS standard has undergone three major developments ranged from V-1.x (collectively known as V-1.0 and V-1.1) to V-3.0. Throughout the developments, more modulation scheme, mainly QAM with different levels and QPSK, and network performance have been accomodated into DOCSIS standards. The overall performance of the various versions is tabulated in Table 2 2, 3. 'DOCISIS with Network Control and Management' Privacy Management of DOCSIS 6 DOCSIS standard only provides data link encryption between CM and CMTS using so called Baseline Privacy Interface (BPI). BPI defines a simple data encryption function that only provides users' data with minimum level of privacy over the HFC network by performing encryption between CM and CMTS. BPI uses so called Cipher Block Chainning (CBC) algorithm, a mode of Data Encryption Standard (DES), to encrypt data frames transmitted in both upstream and downstream paths. RSA public key encryption technique and Electronic Codebook (ECB) are used to secured encryption key exchange between CM and CMTS. The period at which the secret key shared between CM and CMTS can be specified by network administrator as one of the network management parameters. Quality of Service (QoS) 6 DOCSIS V-1.0 only privides users with QoS limited by what is called best effort service where all data packets from CMs are assigned with equal priority and are subjected to the maximum throughput of the network. A CM can be configured so that both upstream and downstream traffics recieve maximum capacity. Since all data packets are prioritise uniformly in DOCSIS V-1.0, QoS can not be ensured in case where the network is hevily congested. DOCSIS V-1.1, however, further enhanced QoS by introducing some new methodologies to optimise network traffic. The new methods introduced in V-1.1 are summarised in Table 3. DOCSIS V-1.1 enhances QoS by introducing different priorities to data packets in Internet traffics so that certain packets, for example network management packets or user data, could be transmitted through the network more quickly even the network is congested. Reference 1 http://en.wikipedia.org/wiki/DOCSIS 2 https://www.nanog.org/meetings/nanog46/presentations/Sunday/Byju_Intro_DOCSIS_N46.pdf 3 http://www.ieee802.org/3/efm/public/jul01/presentations/gummalla_1_0701.pdf 4 Heyaime-Duvergé, C. C., and Vasant K. Prabhu. "Statistical multiplexing of upstream transmissions in DOCSIS cable networks." Broadcasting, IEEE Transactions on 56.3 (2010): 296-310. 5 Ra, Sang-Jung, Dong-Joon Choi, and Namho Hur. "Design and implementation of 6/12MHz switchable QAM modulator based on DOCSIS 3.0." ICT Convergence (ICTC), 2011 International Conference on. IEEE, 2011. 6 Fellows, David, and Doug Jones. "DOCSIS TM cable modem technology."Communications Magazine, IEEE 39.3 (2001): 202-209.